A method for determining the position of an object, a positioning system, a receiver and an electronic device.
The present invention relates to a method for determining the position of an object according to the preamble of the appended claim 1, a positioning system according to the preamble of the appended claim 9, a searcher receiver according to the preamble of the appended claim 17, a searcher receiver according to the preamble of the appended claim 18, an electronic device according to the preamble of the appended claim 19, an electronic device according to the preamble of the appended claim 21, and a computing server according to the preamble of the appended claim 23.
One known positioning system is the GPS system (Global Positioning System) which presently comprises more than 20 satellites, of which 4 or more are simultaneously within the sight of a receiver; for example in Finland, depending on the latitude, even more than 14 satellites can be detected simultaneously, thanks to visibility across the North Pole. These satellites transmit e.g. positioning data of the satellite, as well as data on the time of the satellite. The receiver to be used in positioning normally deduces its position by calculating the transmission time of a signal transmitted simultaneously from several satellites belonging to the positioning system to the receiver. For the positioning, the receiver must typically receive the signal of at least four visible satellites to make it possible to compute the position.
Each satellite of the GPS system transmits a so-called L1 signal at a carrier frequency of 1575.42 MHz. This frequency is also indicated with 154f0, where f0=10.23 MHz. Furthermore, the satellites transmit an L2 signal at a carrier frequency of 1227.6 MHz, i.e. 120f0. In the satellite, the modulation of these signals is performed with at least one pseudo sequence. This pseudo sequence is different for each satellite. As a result of the modulation, a code-modulated wideband signal is generated. The modulation technique used makes it possible in the receiver to separate the signals transmitted from different satellites, although the carrier frequencies used in the transmission are substantially the same. This modulation technique is called code division multiple access (CDMA). In each satellite, for modulating the L1 signal, the pseudo sequence used is e.g. a so-called C/A code (Coarse/Acquisition code), which is a Gold code. Each GPS satellite transmits a signal by using an individual C/A code. The codes are formed as a modulo-2sum of two 1023-bit binary sequences. The first binary sequence G1 is formed with a polynome X10+X3+1, and the second binary sequence G2 is formed by delaying the polynome X10+X9+X8+X6+X3+X2+1 in such a way that the delay is different for each satellite. This arrangement makes it possible to produce different C/A codes with an identical code generator. The C/A codes are thus binary codes whose chipping rate in the GPS system is 1.023 MHz. The C/A code comprises 1023 chips, wherein the iteration time of the code (epoch) is 1 ms. The carrier of the L1 signal is further modulated with navigation information at a bit rate of 50 bit/s. The navigation information comprises information about the health of the satellite, its orbit, time data, etc.
During their operation, the satellites monitor the condition of their equipment. The satellites may use for example so-called watch-dog operations to detect and report possible faults in the equipment. The errors and malfunctions can be instantaneous or longer lasting. On the basis of the health data, some of the faults can possibly be compensated for, or the information transmitted by a malfunctioning satellite can be totally disregarded. Furthermore, in a situation in which the signal of more than four satellites can be received, different satellites can be weighted differently on the basis of the health data. Thus, it is possible to minimize the effect of errors on measurements, possibly caused by satellites which seem unreliable.
To detect the signals of the satellites and to identify the satellites, the receiver must perform synchronization, whereby the receiver searches for the signal of each satellite at the time and attempts to be synchronized and locked to this signal so that the data transmitted with the signal can be received and demodulated.
The positioning receiver must perform the synchronization e.g. when the receiver is turned on and also in a situation in which the receiver has not been capable of receiving the signal of any satellite for a long time. Such a situation can easily occur e.g. in portable devices, because the device is moving and the antenna of the device is not always in an optimal position in relation to the satellites, which impairs the strength of the signal coming to the receiver. Also, in urban areas, buildings affect the signal to be received, and furthermore, so-called multipath propagation can occur, wherein the transmitted signal comes to the receiver along different paths, e.g. directly from the satellite (line-of-sight) and also reflected from buildings. This multipath propagation causes that the same signal is received as several signals with different phases.
The positioning arrangement has two primary functions:
1. to calculate the pseudo range between the receiver and the different GPS satellites, and
2. to determine the position of the receiver by utilizing the calculated pseudo ranges and the position data of the satellites.
The position data of the satellites at each time can be calculated on the basis of the Ephemeris and time correction data received from the satellites.
The distances to the satellites are called pseudo ranges, because the time is not accurately known in the receiver. The pseudo range can be computed by measuring the pseudo range lags between the signals from different satellites. Because time is not known with absolute precision, the position and the time must be found out preferably by iteration of the measured data with a linearized set of equations. Thus, the determinations of the position and of the time are iterated until a sufficient precision has been found with respect to the time and position.
After the receiver has been synchronized with the received signal, the information transmitted in the signal is demodulated to find out e.g. the Ephemeris and time data transmitted from the satellites.
Positioning systems and positioning receivers of prior art are intended for finding out the position of one object only, i.e. the positioning receiver. However, in practice, situations may occur in which it should be possible to determine the direction and distance between one positioning point and an object. For example, when a mother loses eye contact to her child, the mother should be able to find out in which direction and how far the child has gone. In general, when a searcher is searching for an object, it is primarily these direction and distance data and not the absolute coordinates that are significant for the searcher. If such a problem could be solved by using equipment of prior art, a positioning receiver on the object to be found should transmit positioning data to the positioning receiver of the searcher. Thus, the positioning receiver of the searcher could compute the direction vector on the basis of the positioning data of the object and on the searcher. In practice, the accuracy of such a determination is not always the best possible. In both positionings, errors may occur which in the worst case are accumulated upon calculating the direction vector between the positions. Furthermore, this method has the drawback that two different receivers may use signals transmitted from different satellites for their positioning, wherein the significance of non-compatible interference may increase.
The most significant sources of error affecting the calculation of the pseudo ranges include the atmosphere, intentional inaccuracy, multipath propagation, and the receiver. Some of the atmospheric effects are dependent on the frequency to at least some extent. However, atmospheric effects cannot be significantly compensated for in receivers intended for civil use, because for civil use there is only one carrier frequency (L1) available to be received in the positioning receiver. The organization maintaining the GPS satellite positioning system (U.S. Department of Defence) intentionally provides selective availability (SA) of the signals of the satellites, which impairs the accuracy of the positioning. This inaccuracy is induced either by changing the positioning data transmitted by the satellites or by inducing inaccuracy in the clock of the satellite. As a result of multipath propagation, the receiver may be erroneously synchronized with a multipath propagated signal instead of a directly propagated signal. The path propagated by such a signal is longer than that of a directly propagated signal, wherein the positioning is distorted to some extent. Furthermore, errors can be caused by unideal properties of the positioning receiver. For example, errors can be caused by a deviation of the reference clock of the receiver from the GPS time. The measurement results are also distorted by asymmetries on the different receiving channels of the positioning receiver.
Said primary sources of error can further be divided into commonmode errors and non-common-mode errors. Errors caused by atmospheric effects and said selective availability are common-mode errors. These errors can be assumed to be substantially the same in the vicinity of the receiver. Thus, common-mode errors have substantially the same effect on all the positioning receivers in the same area, provided that they receive the signals of the same satellites. Multipath propagation and unideal properties of the receiver are non-common-mode sources of error, wherein these sources of error can also cause different errors in positioning devices in the same area.
It is an aim of the present invention to provide a method for positioning another GPS receiver to be found in an electronic device performing the searching and comprising at least a positioning receiver. The invention is based on the idea that the searching device finds the direction and distance of the object to be searched with respect to the searcher by utilizing the fact that the common-mode errors can be eliminated. Thus, to start the computing, the position of the searcher is used as the initial position of the object to be found. More precisely, the method according to the present invention is characterized in what will be presented in the characterizing part of the appended claim 1. The positioning system according to the present invention is characterized in what will be presented in the characterizing part of the appended claim 9. The receiver according to an advantageous embodiment of the present invention is characterized in what will be presented in the characterizing part of the appended claim 17. The receiver according to another advantageous embodiment of the present invention is characterized in what will be presented in the characterizing part of the appended claim 18. The electronic device according to an advantageous embodiment of the present invention is characterized in what will be presented in the characterizing part of the appended claim 19. The electronic device according to another advantageous embodiment of the present invention is characterized in what will be presented in the characterizing part of the appended claim 21. The computing server according to the present invention is characterized in what will be presented in the characterizing part of the appended claim 23.
Considerable advantages are achieved with the present invention when compared with methods and receivers of prior art. Using the method of the invention, the direction and distance of an object to be searched from the searcher can be determined in a significantly more accurate way than is possible to achieve with methods and systems of prior art.
The present invention will be described in more detail with reference to the appended drawings, in which
FIG. 1 shows, in a reduced block chart, a receiver to be searched and in which the method of the invention can be applied,
FIG. 2 shows, in a principle view, the effect of various sources of error on the determination of the position to be searched by a preferred embodiment of the method of the invention, and
FIG. 3 shows, in a reduced principle view, the positioning system according to a preferred embodiment of the invention.